Liquid-ring pumps are well known, such as, for example, a water-type pump in which a rotating ring of water in an eccentrically disposed chamber has a piston-like action producing suction for pumping either air or water. Pumps of the liquid-ring type are manufactured by the Nash Engineering Company of Norwalk, Connecticut, U.S.A. To operate a liquid-ring pump, the vacuum pump housing is partially filled with working liquid, generally water. A radially vaned impeller is eccentrically disposed with respect to the vacuum pump housing and upon rotation, the working liquid is thrown toward the periphery of the housing where it will form a liquid ring. The liquid ring seals the space between the rotor vanes and the housing. As the rotor rotates, the liquid ring moves away from the hub or rotor central portion thereby increasing the space in the pumping chamber. This, in turn, will draw the medium to be pumped into the chamber through the inlet port adjacent the rotor. As the rotor continues to rotate, the medium, mostly gas present between the vanes of the rotor is compressed by the liquid ring and is expelled through the discharge port. A continuous supply of working liquid is necessary to prevent an increase in temperature in the pump and to replenish the working liquid which is continuously discharged together with the gas through the discharge port. The major advantage of a liquid-ring pump is that it has only one movable part, the rotor. Such liquid-ring pumps have recently been used in connection with a centrifugal pump for the pumping of fiber suspensions such as paper pulp, at medium consistency, that is at about 6-15% solids consistency. Typically, such pumps utilize a separate vacuum pump, piping from the centrifugal pump to the vacuum pump, a separate motor and motor mount for the vacuum pump etc., in order to exhaust the gas which has been separated from the gas containing medium so that the suspension may be effectively pumped by the centrifugal pump impeller.
U.S. Pat. No. 3,230,890 discloses a centrifugal pump for removing gas from low consistency suspensions or from water having either a built-in vacuum pump or an external vacuum pump.
A fluidizing centrifugal pump for the pumping of gas containing medium consistency fiber suspensions having a built-in vacuum pump is disclosed in U.S. Pat. No. 4,776,758.
Various problems have, however, been encountered with the pump in operation today. For example, the air removal capacity has been significantly lower than required, i.e. the vacuum created has not reached a sufficiently high level. Also, the discharge pressure of the vacuum pump has been found to be too low. In some cases, it is desired to introduce the material discharged from the vacuum pump, mainly a mixture containing gas but also some fibers, into the top portion of a mass tower to recover the fibers. If, however, the discharge pressure of the vacuum pump is too low, the pumped material cannot be conveyed to the top of the mass tower, and an additional vacuum pump must be installed for that purpose. Also, the open annular volume in the common wall between the liquid-ring vacuum pump chamber has a tendency to become clogged by the fibers.
In the known pump, the axial gap between the vanes of the vacuum pump rotor and the axially adjacent walls of the vacuum pump housing are not adjustable but are positioned at a distance or clearance of about 0.4 mm. The reason for such relatively large clearance is the fact that there are a number of factors which render it impossible to further decrease the clearance as the various components of the pump are installed on the shaft or around the shaft starting from the drive end of the shaft. Thus, the dimensions of the components affect the clearance. The result of too wide a clearance is, of course, excess leakage and an insufficient vacuum. Another reason for the wide clearance may also be the fact that the shaft of the pump tends to flex somewhat during the operation creating the risk of mechanical contact between the vacuum pump vanes and the housing walls. Thus, the large clearance has been provided intentionally to ensure proper and long lasting operation of the pump.
The pump in accordance with the present invention is designed to eliminate the above mentioned problems. Accordingly, the pump of the present invention is constructed so that the clearance between the rotor vanes from at least one adjacent side wall of the vacuum pump chamber is greater at the tip thereof than near the rotor central portion. This may be achieved by either providing rotor vanes which taper in radial direction from the rotor central portion towards the tip of the vane and leaving the opposed vacuum pump chamber side walls substantially perpendicular to the direction of the shaft or by providing at least one tapered side wall so that the clearance between the rotor vanes and said side wall increases in the direction from the rotor central portion towards the tip of the rotor vanes. Of course, a combination of tapered rotor and tapered side wall is also possible. Also, one or more of the outer edges of the rotor vanes may taper in one direction and one or more outer vane edges may taper in opposite direction so that the distance between the respective surfaces of revolution of the outer edges of the vanes decreases from the rotor central portion to the tips of the vanes. In a further embodiment some of the vanes may have parallel side edges but are of shorter radial length while other vanes are tapered only from about the point which extends in radial direction beyond the length of the first vanes. In other words, the vanes are of different length, possibly alternating long and short and the longer vanes are outwardly tapered only in the area which extends beyond the shorter vanes in radial direction.
In addition, ports for the admission of make-up air for the control of the vacuum pump may be provided at the rear wall of the vacuum pump. By rear wall of the vacuum pump is meant that wall which is located opposite the suction inlet port and, if the pump is used in connection with a centrifugal pump as further described below, the rear wall of the vacuum pump is the wall opposite the air inlet port and distal the centrifugal pump housing.
The vacuum pump of the present invention may also be provided with means for introducing a liquid into the pump for flushing the pump and freeing the pump from fibers which otherwise tend to block the flow path of the pump and to supply working liquid to the liquid ring.
If the pump is used in combination with a centrifugal pump for pumping medium consistency fiber suspensions, the vacuum pump is preferably located on the same shaft as the centrifugal pump impeller behind an intermediate plate separating the centrifugal pump chamber from the vacuum pump chamber. The centrifugal pump impeller may be provided with a rotor with fluidizing blades either within the pump inlet entirely outside the pump inlet or with a combination thereof.